1. Technical Field
The present invention relates to a piezoelectric vibration reed, a piezoelectric vibrator, an oscillator, an electronic instrument, and a radio timepiece.
2. Description of the Related Art
Electronic devices, such as mobile phone sets or a portable information terminal, include a piezoelectric vibrator using crystal or the like as a time-of-day source, a timing source of a control signal or a reference signal source in many cases. The piezoelectric vibrator includes a tuning-fork type piezoelectric vibration reed which is hermetically sealed in a package formed with a cavity.
The package has a structure in which a pair of glass substrates are placed one on top of another and are joined directly in a state in which one of the pair of glass substrates is formed with a depression, so that the depression is functioned as the cavity. Also, the piezoelectric vibration reed includes a pair of vibrating arm portions arranged in parallel to each other and a base portion configured to integrally fix proximal end sides of the both vibrating arm portions in the longitudinal direction, and the both vibrating arm portions are configured to vibrate (pivot) at a predetermined resonant frequency in the direction toward and away from each other from the proximal end sides as origins.
Incidentally, in recent years, downsizing of various types of electronic instruments including the piezoelectric vibrator integrated therein is in progress as represented by mobile phone sets. Therefore, further downsizing is required also for the piezoelectric vibration reed which constitutes the piezoelectric vibrator. Accordingly, for example, with regard to the piezoelectric vibration reed, reducing the length of the base portion to reduce the entire length of the piezoelectric vibration reed is under review.
However, when mounting the piezoelectric vibration reed, it is mounted via the base portion, and if the length of the base portion is reduced, mount performance may be affected. Therefore, the base portion is designed to have a possible shortest length but at the same time, the length of the base portion should be within a range which does not affect the mount performance.
A vibration leak (leak of vibration energy) may occur through the base portion when the piezoelectric vibration reed is operated. The vibration leak may cause increase in a CI (Crystal Impedance), so that it is necessary to inhibit the vibration leak as much as possible. Vibrations of the vibrating arm portions may be stabilized by maximizing the length of the base portion. However, as described above, since the length of the base portion is designed so that the length is minimized within a range in which the stable mount performance can be secured in terms of downsizing, it is required to inhibit the vibration leak without changing the length.
Therefore, for example, a configuration in which notched portions (so-called notches) notched from both sides of the base portion toward the center in the width direction thereof are formed on the base portion between a connecting portion with respect to the vibration arm portions and a mount portion, and a narrow portion formed with the notched portions is formed to be narrower than other portions of the base portion is described in JP-A-2002-261558.
In this configuration, the vibrations excited by the vibrating arm portions are confined on the side of the vibrating arm portions, thereby being inhibited from leaking to the side of the base portion.
Here, in the configuration in JP-A-2002-261558, the deeper the notched portions, that is, the narrower the narrow portion, the more the width of a route through which the vibrations of the vibrating arm portions are transmitted to the base portion is reduced. Therefore, the vibrations may be confined on the side of the vibrating arm portions, and hence the effect of inhibiting the vibration leak is considered to be enhanced.
However, the rigidity with respect to an external impact or the like may be lowered as the width of the narrow portion is reduced. In contrast, when the width of the narrow portion is increased, the sufficient effect of inhibiting the vibration leak as described above cannot be obtained.